In recent years, the JATMA labeling system has been introduced for pneumatic tires for passenger cars, and such tires require superior levels of both low rolling resistance and wet grip performance. In particular, the required level of wet grip performance is extremely high, and a pneumatic tire able to achieve grade a in terms of wet grip performance based on this labeling system has yet to be developed.
It is known that, conventionally, silica was blended in rubber compositions constituting tread portions of pneumatic tires in order to enhance the balance between low rolling resistance and wet grip performance. However, silica readily aggregates due to the presence of surface silanol groups, and because silica exhibits poor affinity for diene rubbers, silica dispersibility can be poor and, in such cases, the effect of modifying the dynamic visco-elasticity properties, such as the loss tangent (tan δ), of a rubber composition cannot be satisfactorily achieved.
As a result, Japanese Unexamined Patent Application Publication No. 2009-091498A proposes enhancing silica dispersibility and reducing rolling resistance (tan δ at 60° C.) by using a rubber composition obtained by blending silica in a solution polymerization-produced terminally modified styrene-butadiene rubber in which terminals are modified with a polyorganosiloxane or the like. It was found that this rubber composition exhibited the effect of reducing rolling resistance, but the levels demanded by consumers in terms of low rolling resistance and wet grip performance was higher and a significant enhancement in balance between low rolling resistance and wet grip performance was required.